Engine starting and starting time can have a significant impact on customer satisfaction. In general, drivers prefer short, consistent, engine crank times combined with low levels of engine vibration and noise. When an engine starts consistent and quick, the driver has a higher degree of confidence in vehicle operation and performance.
Conventional mechanically driven valve trains operate the intake and exhaust valves based on the position and profile of lobes on a camshaft. The engine crankshaft is connected to the pistons by connecting rods and to the camshaft by a belt or chain. Therefore, the intake and exhaust valve opening and closing events are based on the crankshaft position. This relationship between the crankshaft position, piston position, and valve opening and closing events determines the stroke of a given cylinder, e.g., for a four stroke engine the intake, compression, power, and exhaust strokes. As a result, engine starting and the first cylinder to fire are affected in part by the camshaft/crankshaft timing relationship and the engine stopping position. For example, some engine control systems observe several engine position signals that are based on fixed cam/crank timing. This may increase the cranking period because engine position may not be established until a specific location for both cam and crankshaft is identified.
On the other hand, electromechanically driven valve-trains do not have the physical constraints that tie the camshaft and crankshaft together, i.e., there may not be belts or chains linking the camshaft and crankshaft, at least for some valves. Furthermore, full or partial electromechanical valve-trains may not require a camshaft. Consequently, the physical constraints linking the camshaft and crankshafts are broken. As a result, additional flexibility to control valve timing is possible when electromechanical valves are used in an internal combustion engine.
One method to control electromechanical valve operation during an engine start is described in U.S. Pat. No. 5,765,514. This method provides for closing the intake and exhausts valves then allows the starter to crank the engine. If a signal pulse representing crankshaft rotation through 720 degrees has been generated, an injection sequence for each cylinder and a crankshaft position sequence are set. The injection sequence for the cylinders is initialized when a first crankshaft pulse is generated after generation of a first signal pulse representing crankshaft rotation through 720 degrees. The injection sequence and crankshaft position sequence correspond to the position of each cylinder, whereby the opening/closing timing of each intake valve and exhaust valve can be controlled. The cylinders are set to the exhaust stroke, suction stroke, compression stroke, and explosion stroke, respectively.
The above-mentioned method relies on a signal that produces a pulse once every 720 degrees at the same engine location. This signal is necessary to begin injection and the subsequent first cylinder combustion event. In other words, the method is dependent on engine stopping position, sensor orientation, and sensor configuration, to produce signals that are necessary to start the engine. Because of this limitation, the method can increase engine start time depending on the engine stopping location. For example, if the engine stops after the crankshaft location just after the 720-degree signal is generated, the engine will have to rotate through at least 720 degrees before the first cylinder will fire. Further, if the engine stops at a location just before where the 720-degree signal is generated several outcomes are possible. First, depending on the type of sensor selected, the sensor may not be capable of generating a signal when engine speed is low and engine cranking begins. If this occurs, engine cranking will extend beyond 720 degrees before the first cylinder receives fuel. Second, the sensor may generate a signal so that fuel is injected and the engine fires well before 720 degrees of engine rotation, quickly starting the engine. Therefore, the effect of the before-mentioned method is to produce engine crank times where the time period in which an engine is rotating by torque produced by the starter prior to combustion can vary widely.
In addition, the above-mentioned method closes both intake and exhaust valves during crank until the 720-degree signal is observed. Depending on the position of individual cylinders before crank, various amount of trapped air are held in the cylinder until the 720-degree signal pulse is detected, and then the exhaust valves are opened. As the engine rotates under power of the starter each cylinder compresses trapped air. Engine torque and starter current fluctuate as a result of compressing the various amounts of air. Consequently, engine vibration and electrical power consumption increase when compared to an engine having conventional mechanical valves.
The inventors herein have recognized these disadvantages of the before-mentioned approach. The inventors have also considered these limitations and determined that the before-mentioned approach simply focuses on operating cylinder valves based on conventional four-stroke engine operation and crankshaft position. With the exception of closing valves before cranking, the method operates intake and exhaust valves during a start similar to a conventional mechanically driven valve system. The method does not recognize that operation of intake and exhaust valves does not have to assume four-stroke timing during an engine start. Therefore, the approach overlooks opportunities to reduce engine emissions, vibration, and noise.
One embodiment of the present description includes a method for starting an internal combustion engine with electromechanically actuated valves, the method comprising: processing a signal indicative of engine position; after processing said signal setting an intake stroke on a cylinder with sufficient piston downward movement to produce an engine output; and positioning valve timing based on said set intake stroke.
In this way, in one example, electromechanically actuated valves can be activated to improve engine starting and reduce engine cranking time by sequencing them based on engine position.
In other words, the stroke, (e.g., compression, combustion, intake, exhaust), of cylinders can be set, in one example, to produce a first combustion event in a selected cylinder. And, the valves (of that cylinder, and/or other cylinder) can be positioned based on this set stroke to define the firing order. This may reduce engine crank time and the amount of trapped air pumped through an engine. For example, in a four-cylinder engine, two groups of cylinders can have pistons that are in the same location in respective cylinders, (e.g., cylinders 1 and 4, and cylinders 2 and 3). However, one of the two cylinder groups will have sufficient downward piston movement during engine cranking to induct an air-fuel mixture that can produce a desired engine output before the other cylinder group. Since electromechanical valve timing is not based on a camshaft position, a controller can set the valve timing of a cylinder from the group with sufficient downward piston movement so that the engine produces the desired engine output with reduced engine crank duration. In this way, engine cranking time and the amount of air compressed in cylinders during crank may be reduced when compared to the above-mentioned prior art.
The present disclosure may provide several advantages. Namely, it can reduce engine cranking time before a first combustion event in an engine with electromechanical valves.
In addition, engine noise and vibration may be reduced by an engine controller that can choose a cylinder from a cylinder group, in which to carry out a first combustion event, by defining the cycles of cylinders, via valve timing, within the group. If one cylinder of a cylinder group produces more noise during a first combustion event of an engine start, compared to another cylinder of the cylinder group, the controller can simply choose the cylinder that produces less noise during a start, for a first combustion event, to reduce engine noise.
Yet another advantage of the present disclsoure may be reduced engine emissions during a start. Reduced engine cranking time may lower engine emissions because fewer engine pumping events may occur before an engine is started. By reducing the number of pumping strokes before a start, fewer hydrocarbons from previous engine operation may be pumped through a cylinder and into the exhaust.
Note that there are various approaches to identifying engine starting. For example, the engine start can be the period between when an engine begins turning under the power of a starter, until it is rotating at or above a desired idle speed. Another approach is to identify engine starting as the period beginning from key-on until a desired engine speed/air amount is reached.
The above advantages and other advantages and features will be readily apparent from the following detailed description of the embodiments when taken alone or in connection with the accompanying drawings.